1. Field of the Invention
This invention relates to a diaphragm operable and hence functional as a valve by a pressure difference between its front side and its back side, as well as to its fabrication method.
2. Description of the Prior Art
When controlling the operation of an engine or the like, it is generally required to detect pressures at some different points for either opening or closing desired gas flow passages. Rubber-made diaphragms are used for the above-mentioned control.
In the so-called emission gas recirculation system (hereinafter abbreviated as "EGR" for the sake of simplification) of a car engine for example, so-called EGR modulator 4 and EGR valve 5 are provided between an intake side 1A of an engine 1 with which an intake system 2 including a carburetor communicates and an outlet side 1B of an engine 1 leading to an exhaust system 3 as shown in FIG. 1. The EGR modulator 4 is equipped with a diaphragm 6, which is normally separated by the action of a spring (not shown) from an intake port 7 communicating with the intake side 1A so that the intake port 7 is normally kept open to maintain the intake side 1A in communication with an air intake port 8 which leads to the atmosphere. When the pressure in the intake side 1A has dropped and the pressure in the outlet side 1B has conversely gone up, the diaphragm 6 is pushed upwardly to close the intake port 7 so that the introduction of air from the atmosphere becomes no longer available. On the other hand, the EGR valve 5 has a diaphragm 9. This diaphragm 9 generally closes an intake port 10 which communicates with the outlet side 1B. When the pressure in the intake side 1A and outlet side 1B have respectively decreased and increased and further intake of air has been stopped by the EGR modulator 4, the diaphragm 9 is however pushed upwardly due to a pressure difference and is thus separated from the intake port 10. This causes the exhaust port 10 to open and hence to communicate to a bypass 11, thereby recirculating a portion of the exhaust gas in the outlet side 1B to the engine 1 by way of the bypass 11 and intake side 1A. 12 denotes a vacuum line.
Such diaphragms, for example, those suitable for use in EGR modules similar to the EGR modulator 4 have hitherto been fabricated for example in the following manner. As shown in FIG. 2, a fluorosilicone rubber was molded and then cured under heat to form a diaphragm main body 22 equipped with a ring-like deformable portion 21 having an arched cross-section. On the side, a fluorine-containing rubber that is, a fluorocarbon rubber, was molded and then cured under heat to form a valve member 23. Then, the valve member 23 was centrally disposed on one side of the diaphragm main body 22, and a retainer plate 24 was provided on the same side of the diaphragm main body 22 to bring the retainer plate 24 into engagement with a flange portion 23a of the valve member 23. In addition, a backup plate 25 was arranged on the other side of the diaphragm main body 22. Thereafter, the valve member 23 and diaphragm main body 22 were held under pressure between the retainer plate 24 and backup plate 25, for example, by causing pins 26 to extend from the retainer plate 24 and through the diaphragm main body 22, and then connecting the pins 26 to the backup plate 25. Numeral 27 indicates a peripheral mounting part of the diaphragm main body 22.
Fluorosilicone rubber is used as a material for the diaphragm main body 22 for the following reasons. This rubber is deformable to a significant extent and, particularly, maintains sufficient deformability even at low temperatures. In addition, it has outstandingly high anti-deformation durability and chemical stability. On the other hand, fluorine containing rubber is used to make up the valve member 23 for the following reasons. Namely, fluorine-containing rubber exhibits very high abrasion and wearing resistance and at the same time, enjoys chemical stability. However, the deformability of fluorine-containing rubber becomes extremely small especially at low temperature, thereby increasing its stiffness. Therefore, fluorine-containing rubber cannot be used as a material for the deformable portion and fluorosilicone rubber cannot be employed as a material for the valve member due to its low abrasion and wearing resistance.
In a diaphragm of the above-mentioned sort, it is necessary to use both fluorosilicone rubber and fluorine-containing rubber in order to simultaneously satisfy both of the required two functions, specifically, the deformability as a diaphragm and durability as a valve member. They cannot however be cured at the same time because their curing systems are different from each other. In the past, it was necessary that the diaphragm main body 22 of a fluorosilicone rubber and the valve member 23 of a fluorine-containing rubber were separately molded and then cured. They were then put together by holding means such as the retainer plate 24 and backup plate 25. That conventional fabrication method was thus accompanied by drawbacks such as a plurality of rubber-made parts had to be completed individually, thereby requiring many fabrication steps; due to need for holding means, the final products became unavoidably heavy; and there was a danger that their airtightness would become insufficient depending on the condition of the connection by the holding means.